1. Field of Invention
The invention relates to satellite communications and demodulation and decoding of digital data.
2. Prior Art
The worldwide utilization of satellite communications for the reception of digital TV signals has grown at a rapid pace in the last 10 years. The utilization of digital video technology for Direct Broadcast Satellite (“DBS”) systems allowed many folds increase in the number of video programs that can be transmitted over a single satellite and significantly accelerated the utilization of satellites for video transmission directly to the consumer both in the US and internationally.
A typical DBS home installation includes one or more dish antennae that receive signals from one or more broadcasting satellites. Some dishes support multi-satellite feeds and can receive signals from more than one satellite (usually satellites in adjacent orbital locations or at different frequency bands). In this typical installation, multiple Low Noise Block Converters (LNBs) accept the high radio frequency (RF) satellite signal feed, down covert the signals to an intermediate frequency (IF) signal, drive a multi-port RF switch, and the switch outputs are connected through cables to the various Set-Top Boxes (STBs) inside the house.
FIG. 1 shows a typical configuration where all the available and relevant satellite signals are presented at the multi-port switch inputs. Each STB in turn, selects the specific satellite signal it needs to view a desired program by monitoring an internal look-up table guide that lists the specific satellite, polarization and transponder required for reception of the program. A single transponder may carry several video programs. A satellite transponder signal is also called a channel. A control signal is sent by the STB to the switch through the connected cable to select the desired switch input that contains the desired program. Once a selection is made, the switch will make the input available on the cable for demodulation and decoding by the STB.
A typical DBS satellite carries two polarizations with 16 transponders each. Since each of the satellite polarizations provides 500 MHz bandwidth, the total bandwidth of a DBS satellite is around 1 GHz. This 1 GHz of bandwidth is typically configured in two primary ways. In one configuration, there are one or two outputs from the LNB; in the case of one output, only one polarization is selected by the STB and is provided to the LNB output, while in the case of two LNB outputs each LNB output carries a single polarization with bandwidth of about 500 MHz. FIG. 2 shows transponder frequencies for the case where there are two LNB outputs. Typically, the signals from the two polarizations are offset in frequency such that the center frequency of the transponders in one polarization falls in between transponder frequencies of the other polarization.
FIG. 3 shows a second configuration, where the two polarizations are “stacked” together and are contained in the 950-2150 MHz band. Since a reasonably priced, long inter-facility link (IFL) cable may be able to carry less than 1.5 GHz of total bandwidth above 950 MHz, at most only two 500 MHz wide polarizations can be carried on the cable. The 500 MHz signals can be from the same or different satellites. The frequencies from 0 to 760 MHz or 850 MHz are commonly used for over the air broadcast frequencies or cable television.
The prior art architecture needs to be changed drastically once a single cable STB requires the simultaneous reception of more than two programs. Since the multiple programs may be sourced from different satellites and/or different polarizations, the total bandwidth required to receive all the programs can be as much as 500 MHz per each viewed program for the number of simultaneously received programs, which can easily exceed the tuning bandwidth available on a single IFL cable, and could be as high as the total bandwidth of all the satellites. This is particularly a problem with the anticipated wide deployment of Digital Video Recorders (DVRs). The ability to record multiple simultaneous programs is critically important for satellite operators since it can mitigate somewhat their inability to offer a true Video on Demand (VOD) services offered by competitors such as the cable operators. The ability to record multiple simultaneous programs enables the user to have an effective time shifting alternative to VOD. Thus, it is quite advantageous for the satellite operators to provide a STB with several channel receivers, thus enabling the user to watch a program while simultaneously recording several others. Also, one can consider an architecture where a “Media Center” STB with multiple receivers feeds can provide multimedia programming such as video and audio to multiple “Thin Client” STBs in other rooms using home networking technology for the transmission of the selected programming.
More recently, new satellite switches have been developed that can place a number of selected individual satellite transponder channels from all the connected satellites and polarizations into a single IFL cable, as disclosed in U.S. Pat. No. 7,130,576 issued Oct. 31, 2006 entitled “Signal selector and combiner for broadband content distribution”, subject to common assignment as the present application and incorporated herein by reference.
FIG. 4 shows the new system where a single STB connected to the outdoor unit (ODU) by a single IFL cable can now receive programs from two or more separate satellites simultaneously. This enables an STB connected by a single cable to employ two or more tuners and have simultaneous access to multiple programs, independent of how many satellites are being received. This is very important since the STB may serve two TV sets simultaneously or it enables the user to watch one program and at the same time record a second program from any of the connected satellites.
In order to receive multiple simultaneous programs from multiple satellites in a STB with a single IFL cable, additional processing of the signals from the various LNBs is performed. Transponder signals are selected from the LNB outputs and combined into a composite signal to enable all the desired channels requested by the various STBs to be processed and provided on the single IFL cable. More than a single IFL cable can be utilized if necessary, whereas the necessary channels are delivered to the corresponding STBs connected to each cable.
There are several alternative options that can be considered but all deal with extracting portions of the traditional satellite full-polarization signal to be transmitted through the IFL cable. Following is a short discussion of the alternatives:
1. For each of the desired programs, extracting a portion of the signal from the LNB that includes the transponder carrying the program. Then constructing an equivalent IFL signal that includes all the newly extracted signals that carry all the desired programs. In this fashion, assuming that one does not need to perform any signal demodulation, the minimum signal section that is required to carry a given program is a single bandwidth section that carries the desired program, which is a transponder channel. Assuming an IFL capable of carrying the traditional 950-2150 MHz bandwidth, as many as 32 arbitrary programs from multiple satellites can be received simultaneously. The outdoor device performs the function of selecting sections of multiple satellite polarizations and reconstructing a new signal for transmission to the indoor unit is called generically a Frequency Translating Module (FTM). The FTM can be constructed in many configurations, but in the present document it denotes a device that selects several desired transponder channels and reconstructs these channels into a certain span of bandwidth (usually in the 950-2150 MHz band) for inputs to the STBs.
2. Other alternatives that can carry even more programs exist, but may require more elaborate signal processing performed outdoors. In this case, the digital video and audio signals of the desired program are extracted from the corresponding transponders and transmitted to the STBs for storage and/or viewing, or for alternate processing including storage in other devices. The aggregate signals that include all the digital video and audio signals of the selected program can be carried in several forms including various forms of modulation different than the original modulation (denoted trans-modulation), or where the various digital video signals of the selected programs are re-multiplexed, and re-modulated in a fashion similar to the original signals and then can be received by the same STBs that receive the original signals, or can be delivered as digital signals over some networking technology and protocol such as Ethernet or others.
Once all the selected transponder channel signals have been re-arranged to fit in a single cable, it enables another highly beneficial option. Since all these transponders have been selected based on user requirements, it is assumed that all, or at least most of them are arranged in a contiguous fashion in the frequency domain. Hence, all the channels that need to be demodulated are available in a certain section of bandwidth that can be fit within the bandwidth carrying capability of a single cable. Hence, these channels can be demodulated simultaneously utilizing a new broadband demodulator front-end described below.
Traditional multi-channel DBS system reception requires the selection of the desired programs and the corresponding one or more transponder channels that carry these programs to be demodulated simultaneously. However, since the selected programs are arbitrary, the corresponding transponder channels can be at arbitrary frequencies, transmitted from arbitrary satellites (in a multiple satellite system) and at arbitrary polarization.
In the traditional method, a transponder channel is received from a selected satellite and a selected polarization, and a separate tuner/demodulator receives the transponder channel frequency and demodulates it. Hence, it requires a separate tuner and demodulator for each demodulated transponder channel and the duplication of the signal processing required for demodulating and decoding all the channels.
If, however, all the required transponder channels are available in a certain range of frequencies at a given polarization, as is the case discussed above where the system organizes all the transponder channels that require demodulation to be available on such a bandwidth section, then it is possible to utilize a single broadband tuner to select and down convert the corresponding section of bandwidth before the analog to digital (A/D) conversion process used by the digital demodulator(s). Also, the demodulator(s) can utilize a broadband front-end to digitize the entire section of bandwidth and perform the remaining demodulation and signal processing in the digital domain that could be more efficient compared to the multiple simultaneous demodulators. One approach is described in Zhang, et al, Digital implementation of multi-channel demodulators, U.S. published application 20030056221, published Mar. 20, 2003.
FIG. 5 shows prior art, multiple demodulator architecture in existing satellite STBs. In this implementation, RF signals from one or more cables in the 950-2150 MHz range are provided as input to multiple tuners. Depending on system configuration, the RF input signals to the tuners can be the same to all tuners, different to each tuner or any other configuration where the RF input to some of the tuners is the same. Each of the tuners tunes to the transponder frequency that contains the desired video channel and provides I and Q baseband components to the demodulator/decoder for demodulating, decoding and data extraction that is provided to the transport stream decoding, conditional access decryption and MPEG decoding. This architecture is very flexible and can receive and decode any channel from any satellite, polarization and transponder assuming that the system can provide access to all satellites and polarizations (through the utilization of an appropriate outdoor switch).
FIG. 6 shows a block diagram of a prior art tuner, such as one of those shown in FIG. 5. In this block diagram, a single conversion tuner is shown. The RF input signal is amplified by a front-end low noise amplifier (LNA) with gain and gain control. The gain control is needed to compensate for the potential large dynamic range that can be encountered due to different IFL cable attenuations and LNB gain. The RF signal is split and then down-converted by a Quadrature down-converter. The down converted components, designated as I and Q, are each filtered by a baseband filter and then amplified by a baseband Automatic Gain Control (AGC) and provided to the demodulator/decoder. The whole process is generally controlled by a controller that selects the synthesizer frequency (mostly as a function of user channel selection), adjusts the RF AGC levels (if necessary, the AGC can be an independent function as well or in combination with the controller) and baseband filter bandwidth.
FIG. 7 shows the spectrum of the signals of FIG. 6. One of the transponder channels (Ch k) is selected for demodulation by the controller for viewing one or more of the digital video signals transmitted in this transponder channel. The synthesizer is tuned to the center frequency of Ch k and the quadrature down-converter down converts the Ch k signal to the two baseband components, Ch k-I and Ch k-Q centered at 0 Hz or other low frequency. The low pass I and Q filters filter the down-converted I and Q signals. The signals are amplified appropriately by fixed or variable gain amplifiers and passed to the demodulator/decoder for all the additional processing needed in order to extract the desired digital video channel.
The main functions performed after the down conversion and digitization of the I and Q signals are the demodulation of the transponder signal and performing the Forward Error Correction (FEC) decoding. Various techniques of demodulation and FEC decoding are well known and are widely discussed in numerous books and technical literature. For example, “Digital Communications” by John Proakis, published by McGraw Hill series in Electrical and Computer Engineering, discusses extensively various techniques for demodulating and decoding digital communication signals.
Error correction coding and decoding is needed for the transmission and reception of digital video signals. To conserve transmitter power and reduce the size of the receiving antenna dish, the links are being operated with low Signal to Noise Ratio (SNR) margins and non-zero raw bit error rates (BER). Digital video requires near error free data to avoid visible impairments to the viewed program. Corrected error rates of 1 bit in a billion (1e-9), or better, are typically needed to result in a satisfactory video image. Compression of the digital data increases the need for error free data. Well-known Forward Error Correction (FEC) techniques are used to assure error free communication in the noisy channel. Redundancy or check bits are added to the transmitted video data then used to correct errors in the data in the receiver. In satellite systems, such as the one specified by the Digital Video Broadcasting for Satellite (DVB-S1) standard, error-correcting block codes such as Reed-Solomon (RS) codes are used in conjunction with convolutional codes. In this concatenated coding approach, the RS code is the outer code and the convolutional code is the inner code. A two stage decoding process is done in the receiver.
More powerful FEC techniques such as turbo code or low density parity check (LDPC) code FEC can be applied to satellite communication transmission techniques to further improve BER at a given power level or reduce power requirements while maintaining the same BER as other FEC approaches. Turbo codes, coders, and decoders are described in U.S. Pat. No. 5,406,570, Berrou et al., Apr. 11, 1995, entitled “Method for a maximum likelihood decoding of a convolutional code with decision weighting, and corresponding decoder”; U.S. Pat. No. 5,563,897, Pyndiah et al., Oct. 8, 1996, entitled “Method for detecting information bits processed by concatenated block codes”; U.S. Pat. No. 6,304,996 Van Stralen et al. Oct. 16, 2001, entitled “High-speed turbo decoder”, each incorporated herein by reference. LDPC codes, encoders, and decoders are described in R. G. Gallager, “Low-Density Parity-Check Codes”, M.I.T. Press, 1963; U.S. Pat. No. 6,961,888, Jin et al, Nov. 1, 2005, entitled “Methods and apparatus for encoding LDPC codes”; U.S. Pat. No. 6,633,856 Richardson et al, Oct. 14, 2003, entitled “Methods and apparatus for decoding LDPC codes.” Turbo and LDPC codes and decoders are well known.
Turbo and LDPC codes have a disadvantage of high decoding complexity. Turbo and LDPC decoders operate iteratively on the received data block to correct errors and the number of iterations required to decode the data is not known with certainty until the decoding process is complete. A system must have sufficient processing power to iteratively decode the data under worst-case expected conditions. The decoder power needed for the worst-case condition is higher than the average requirement.
A system may include a large buffer for a single stream of data of incoming data followed by a single iterative decoder. While the decoder is operating on a block taking longer than average, the buffer stores incoming data. This allows some averaging of processing demands on a single stream of data over time, at the expense of latency and large buffer capacity.
The prior art approaches to processing multiple channels of digital video data rely on separate error correction units for each channel. Each decoder needs processing power equal to the worst-case requirement. A need exists for efficient processing of several transponder channels in a set top box.